1. Field of the Invention
The present invention relates to an extrusion-type coating method and coating apparatus, particularly to an extrusion-type coating method and coating apparatus for coating a web with coating liquids to form thin and uniform magnetic recording layers in order to produce a magnetic storage medium with a high capacity and a high density.
2. Description of the Related Art
In order to produce a magnetic storage medium with a high capacity and a high density, a thin and uniform magnetic recording layer needs to be formed on a web. In an extrusion-type coating apparatus, the continuously-running web is relatively pressed against edge faces of a coating head, and one or more coating liquids supplied into the coating head are supplied to discharge mouths of the coating head through supply routes and discharged from the discharge mouths of the coating head formed in the width direction of the web, so as to coat the web with the coating liquids. Moreover, an extrusion-type coating method is performed by which plural coating layers having a first layer (bottom layer) as a non-magnetic layer and a second layer (top layer) as a magnetic layer are simultaneously formed, and an extremely thin magnetic recording layer is thus obtained.
In such extrusion-type coating method, a coating defect may occur such as an unevenness of a thickness of the magnetic recording layer in the width direction of the web if the web is coated unevenly in the width direction of the web. The higher the density of the magnetic recording layer is, the higher the required precision of the uniformity is.
In a conventional art, improvements in a form of the edge face, a tension condition of the web, and further a positional relationship of each of the edge faces with respect to the web surface, are attempted in or to attain the uniformity of the magnetic recording layer. Some of such prior arts are Japanese Patent Application Laid-open Nos. 60-238179, 62-132566, and 4-145977, in which discussions are described on properly setting a thickness of a back blade, on properly setting an entering angle of the web into the coating head and on adjusting a space between a trailing end of the edge face and the web. Further, Japanese Patent Application Laid-open No. 6-296917 teaches that a coating head which can be divided into plural blocks for each of the edge faces and a level difference is formed on each of the edge faces by sliding each block, whereby the positional relationship between the edge faces with respect to the web surface is appropriately set. Still further, Japanese Patent Application Laid-open No. 11-314065 teaches that a length of the perpendicular is adjusted in the width direction of a web, in which a leading end on an edge face of a block at the trailing end becomes A, and a trailing end on the edge face of the block at the leading end becomes B, the perpendicular being connected with a tangent line, that is written on the edge face of the block at the trailing end from the trailing end B on the edge face of the block at the leading end by passing a supply route.
When assembling a coating head in a method for setting level differences of the edge faces by using the coating head which can be divided into plural blocks for each of the edge faces mentioned above, each block is processed beforehand in a size with a height suitable for a level difference so that the level difference has a desired height, then the blocks are assembled and fastened while being placed on a reference plane. In another method, the heights as well as sizes of the blocks are measured beforehand when assembling the coating head, and the heights of the blocks are adjusted by laying a spacer at the bottom of each block and the blocks are fastened in that state. In short, the level differences of the edge faces are set by referring to manufacturing precision and flatness as a reference of the blocks.
However, when setting the level difference by the above-described method, a magnetic recording layer which is satisfactory uniform cannot be obtained if the manufacturing precision of the blocks is low, because the precision of the levels is determined by results of the manufacturing precision of the blocks. In a case where the width of the web is wide such as 1 meter or more, a technology for manufacturing the blocks with precision of a few micrometers is extremely cost-consuming as well as difficult.
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of an extrusion-type coating method and apparatus in which level difference of edge faces can be precisely set to form a uniform layer, and unnecessarily-high manufacturing precision of the blocks is not needed to avoid an increase of costs for block manufacturing.
In order to achieve the above-described objects, the present invention provides an extrusion-type coating method wherein a continuously-running web is relatively pressed against edge faces of a coating head, a coating liquid supplied into the coating head is supplied to a discharge mouth formed on the edge faces in a direction of a width of the web through a supply route and is discharged from the discharge mouth so as to apply the coating liquid on the web, and the coating head is divided into two blocks along the supply route so that a level difference between the edge faces can be set, the extrusion-type coating method comprising the steps of: preliminarily setting the level difference by assembling the blocks on a base with a reference plane so that the level difference between the edge faces is a desired level difference; then measuring a variance of the level difference preliminarily set in the width direction of the web; then finally setting the level difference according to results of the measurement; and then applying the coating liquid on the web by using the coating head into which the set blocks are assembled.
Further, in order to achieve the above-described objects, the present invention provides an extrusion-type coating apparatus wherein a continuously-running web is relatively pressed against edge faces of a coating head, a coating liquid supplied into the coating head is supplied to a discharge mouth formed on the edge faces in a direction of a width of the web through a supply route and is discharged from the discharge mouth so as to apply the coating liquid on the web, and the coating head is divided into two blocks along the supply route so that a level difference between the edge faces can be set, the extrusion-type coating apparatus comprising: a level difference variance measurement device which measures a variance of the level difference in the width direction of the web; and a level difference correction device which corrects the level difference according to results of the measurement by the level difference variance measurement device so that the variance of the level difference is reduced.
Accordingly, the extrusion-type coating method and apparatus of the present invention first forms the edge faces with the level difference by assembling the blocks constructing the coating head, and measures the variance of the level difference in the width direction of the web by the level difference variance measurement device, then adjusts the level difference to eliminate the variance with reference to the measured results. Thus, a uniform coating layer can be formed on the web. Furthermore, the coating layer can be still thinner because the coating layer can be uniform.